1. Field of the Invention
The present invention relates to a process for controlling a distribution of braking force in a vehicle for front and rear wheel brakes by regulating a braking liquid pressure for at least the rear wheel brakes based on a comparison of a wheel speed difference target value and a difference between front and rear wheel speeds.
2. Description of the Related Art
Techniques are conventionally know, from Japanese Patent Publication No. 51-26584, Japanese Patent Application Laid-open No. 6-144178and the like, in which a front and rear braking-force distribution control is carried out so as to eliminate the difference between the front and rear wheels, thereby achieving an idel braking-force distribution.
In the above know techniques, optimal braking forces, corresponding to loads applied to the front and rear wheels, are provided by controlling the front and rear braking-force distribution so as to eliminate the difference between the front and rear wheel speeds. This is based on the following principle: When a braking force is generated for a tire receiving a vertical drag force N from a road surface, a slip rate 80 of the tire is represented, for simplicity, in a very small region by the following equation: EQU .lambda.=K.times.(B/N) (wherein K is a constant)
The ideal braking-force distribution is to ensure that the front and rear wheel braking forces are proportional to the vertical drag force. Therefore, the front and rear wheel slip rates 80 may be equalized. Thus, if the vehicle speed is represented by V.sub.v and the wheel speed is represented by V.sub.w the slip rate .lambda. is determined according to the following equation: EQU .lambda.=(V.sub.v -V.sub.w)/V.sub.v
Therefore, if the braking-force distribution is controlled such that the front and rear wheel speeds VW become equal to each other, the front and rear wheel slip rates .lambda. can be equalized. More specifically, the rear wheel speed V.sub.w is controlled such that the rear wheel speed V.sub.w becomes equal to the front wheel speed V.sub.w.
It is difficult to directly measure the wheel speeds V.sub.w. In the existing circumstances, a rotational speed .omega. of a tire is detected by a rotational speed sensor, such as an electromagnetic pick-up. If a dynamic radius of the tire is represented by r, a wheel speed V.sub.w is calculated according to the following equation: EQU V.sub.w =r.times..omega.
However, the dynamic radius r varies depending upon various conditions such as air pressure in the tire, load, vehicle speed and the like. Therefore, unless the dynamic radius r is connected for the variations in the conditions this method fails to provide a highly accurate wheel speed V.sub.w.
A technique is known in which the correction of the wheel diameters is carried out during traveling of the vehicle at a constant speed, before the control of the front and rear braking-force distribution is carried out. If the correction is only carried out in this manner however, it is impossible to accommodate the variation in dynamic radius r due to the displacement of the load during speed-reduction due to braking. That is, during speed-reduction of the vehicle, the dynamic radius r is smaller for the front wheel and larger for the rear wheel due to the displacement of the load caused by speed-reduction. Hence, there is a possibility that the slip rate of the rear wheel is apparently large relative to the slip rate of the front wheel, and the braking-force distribution ratio for the rear wheel is too small relative to an appropriate value as a result of the control of the front and rear braking-force distribution.